1. Field of the Invention
This invention pertains to the field of telegraphy. More particularly, this invention pertains to television systems. In still greater particularity, this invention pertains to a shutter synchronizer system. By way of further characterization, but not by way of limitation thereto, the invention utilizes a shutter servo system to expose the video tube of a television camera for a fixed period during the camera's vertical retrace time.
2. Description of the Prior Art
In a conventional video camera, incident light continuously exposes the light sensitive material on the face of the video tube. Some electrical property, usually the conductance, of this light sensitive material is changed in accordance with the exposure of the face to the image. This changed property is in turn sensed by a scanning electron beam so that the potential at the output grid of the tube varies in accordance with the amount of light that exposes each point. The electron beam also restores the light sensitive material to its unexposed condition.
In a standard video camera, the electron beam scans the entire face each 16.67 msec. with 243 equally spaced lines producing a field. Thus, the exposure time of each point on the face of a standard video camera is 16.67 msec. with each point on the face being scanned and erased at a different time. In order to provide greater clarity the electron beam in a standard camera produces two interlaced fields. The two fields constitute a frame. The frame is changed at a rate which, due to the persistence of vision, results in the illusion of motion. These two facts make a standard video camera unsuitable for filming high speed events, that is, movement between field scans results in a blurred image due to object displacement.
If a light shutter is placed in front of the face of the video tube the exposure time can be reduced to as low as 0.1 msec. The electrical impression of the image remains until scanned by the electron beam. With such a shutter high speed events may be filmed. However, if the shutter is not synchronized with the scanning electron beam, then exposures will occur during the time the face is being scanned. Exposure during the scan period results in blurred or multiple images and renders non-synchronized shutters less than satisfactory for filming high speed events.
It would be desirable to have a shutter synchronized to expose the face of the video tube only during the time between scans of the electron beam, herein called the vertical retrace period. This allows for filming of high speed events. While shuttered video cameras have been available for years, they require manual synchronization and, once synchronized, they may lose their synchronization during use resulting in unsatisfactory pictures. A system providing automatic, stable, and very precise synchronization is highly desirable.